LibJS: Add NativeFunction, a callable wrapper around a C++ lambda

This can be used to implement arbitrary functionality, callable from
JavaScript.

To make this work, I had to change the way CallExpression passes
arguments to the callee. Instead of a HashMap<String, Value>, we now
pass an ordered list of Argument { String name; Value value; }.

This patch includes a native "print(argument)" function. :^)

Libraries/LibJS/Object.h

@@ -42,6 +42,7 @@ public:
     void put(String property_name, Value);
 
     virtual bool is_function() const { return false; }
+    virtual bool is_native_function() const { return false; }
 
     virtual const char* class_name() const override { return "Object"; }
     virtual void visit_children(Cell::Visitor&) override;